Hydrogen peroxide is commercially produced using a process known as the Riedl-Pfleiderer process. In accordance with this two step process, anthraquinone in a carrier solvent, termed "working solution", is cycled between an oxidation reactor and a hydrogenation reactor to convert hydrogen plus oxygen to hydrogen peroxide. Variations to the process have concentrated on the form of anthraquinone, the composition of the working solution and the type of catalyst used. A typical catalyst is palladium, raney nickel, or nickel boride on an inert support. The catalyst may be in the form of a slurry or a fixed bed. Hydrogen is needed at high partial pressures in this reaction posing the risk of explosion. The process is characterized as being complex and capital intensive.
Processes for the direct oxidation of hydrogen and oxygen to hydrogen peroxide offer the opportunity to develop a simpler, less costly process. Processes of this nature have been proposed, but to the inventors' knowledge have not been commercialized to date. The difficulties with the heretofore proposed processes include:
--low concentrations of product PA1 --low selectivity (thus high hydrogen consumption) PA1 --low reaction rates PA1 --hazardous operating conditions (particularly hydrogen partial pressure requirements in the explosive range) and PA1 --high acid content. PA1 (a) contacting the hydrogen and oxygen containing acidic aqueous medium with a catalyst consisting of at least one Group VIII metal on a partially hydrophobic, partially hydrophillic support in a pressure vessel; PA1 (b) providing a source of sodium and chloride ions to the acidic aqueous medium either at the outset of the reaction or once there is a decline in catalytic activity; PA1 (c) maintaining the pressure in the vessel in the range of 3.5 MPa-20 MPa, with a hydrogen partial pressure below the explosive limit; and PA1 (d) maintaining the temperature in the range of the freezing point of the aqueous medium to about 60.degree. C. PA1 (a) a partially hydrophobic, partially hydrophillic support, preferably Vulcan carbon or fluorinated carbon with a 10-65% F content; PA1 (b) a Group VIII metal; and PA1 (c) a source of sodium and chloride ions. PA1 (a) providing sodium citrate and a Group VIII metal salt in an aqueous solution; PA1 (b) heating the solution to form a Group VIII--sodium citrate colloid; PA1 (c) adding a catalyst support to the colloid containing solution; PA1 (d) evaporating the solution from the solid; and PA1 (e) reducing the resulting solid in a hydrogen atmosphere.
Exemplary of such processes are the following patents, all of which include catalytic conversion of hydrogen with oxygen in an acidic aqueous medium:
U.S. Pat. No. 4,009,252 issued to Izumi et al. reports good product concentrations (9-12% H.sub.2 O.sub.2 by wt.) by operating at high acid concentrations (1 gpl HCl plus 49 gpl H.sub.2 SO.sub.4) using Pd deposited on silicic acid, and oxygen to hydrogen molar ratios of 1.5 to 20, well into the explosive range for hydrogen. Selectivities for hydrogen to hydrogen peroxide were good with many examples in the range of from 80-89%. Reaction rates were generally low, ranging from less than 1 to just over 6g of hydrogen peroxide per liter-hour.
U.S. Pat. No. 4,661,337 issued to Brill reports high concentrations of hydrogen peroxide and high reaction rates using Pd deposited on carbon in an aqueous solution containing 35 gpl HCl, by operating a stirred reactor in such a manner to keep the thickness of the aqueous slurry to 2 mm or less. For example, concentrations of 19.5% hydrogen peroxide were achieved at a rate of 48g of hydrogen peroxide per liter-hour using hydrogen at 250 psi partial pressure and oxygen at 750 psi total pressure. However, much of the benefit of the higher reaction rates was lost since most of the reaction vessel was empty. Also, the reaction conditions were in the explosive range for hydrogen.
U.S. Pat. No. 4,772,458 issued to Gosser et al. (see also U.S. Pat. No. 4,681,751 and EPA 0132294 to Gosser et al.) achieved high concentrations and reaction rates with moderate selectivity at low acid levels (less than 2.5 gpl H.sub.2 SO.sub.4) using Group VIII metals on a variety of carriers, but at hydrogen concentrations of 17% or higher, making the process hazardous. Selectivities tended to be low, ranging from 30% to 70%, provided bromide ions were present in the reaction medium. If chloride ions were used, very low selectivities of about 6% were achieved. The best results appear to have been achieved using a 1:10 ratio of Pt to Pd on an alumina carrier (1.10% total metal) with a hydrogen concentration of 17.8%. Hydrogen peroxide concentrations were 16.4% at 70% selectivity and the reaction rate was 52g hydrogen peroxide per liter-hour.
There is a need for a direct oxidative process for the production of hydrogen peroxide which will produce hydrogen peroxide in good concentrations and at high selectivities and reaction rates, while allowing the process to be conducted at low acid levels and below the explosive range of hydrogen.